Well, this may seem counterintuitive but three pieces of advice:
1. but if you are really having fit problems and so forth, then the REI is a good place to go. They carry just about everything and really care. They have a compete set of orthodics and things as well.
2. There is a great guy at the Pro Club who fits folks with really hard problems with feet. I can get the guys name.
3. If you have general good feet and like the brand you use, then shoebuy.com is a great place. This is online. the great secret of running shoes is that although the models change every 10 minutes, the basic sizing and so called Last (the shape of the shoe doesn't). So if you find a shoe type that you like you can stick with it. For me, the Asics has been that brand. You can look in running magazine (or their site) for some samples.